DEVELOPMENTAL HISTORY OF THE MOLLUSCA. dl 
Fig. 21 is a day younger than fig. 22 ; the former gives a profile view, the latter is seen 
from the surface with the aperture of invagination uppermost. ‘The aperture (7) closes 
entirely a day later, and the deyelopment of the true mouth proceeds at another spot 
stilllater. ‘The sharp distinction between the invaginated cells (Ay) and the thick layer 
of smaller epiblastic cells (ep) makes this Gastrul/a form one of the most typical among 
Mollusca. 
Limneus stagnalis *—A\though I am unable to present at this time any drawings of 
the development of this common Pulmonate, I must yet point out that it is one of the 
most interesting and important in relation to the two new features of molluscan deve- 
lopment pointed out in these contributions, viz. the invaginated Gastrula-phase and the 
rudimentary shell-sac and plug. 
In Limneus stagnalis a Gastrula is developed by invagination, which is one of the 
best marked in all the animal kingdom. Its aperture of invagination has been mistaken 
by Leresou.ier (who has well figured it without, as may be supposed, appreciating its 
significance) for the mouth. 
Similarly in the same mollusk, at a later stage, a thickened “shell-patch ” develops, 
which exhibits a very deeply marked groove or pit, the shell-groove. This has also been 
seen and figured by LEREBOULLET, who has mistaken it for the commencing invagination 
of the anus. 
Thus Limneus presents these two important developmental features in a strongly 
marked condition. 
* January 7, 1875.—The above was written in January 1874, and the facts to which it refers were 
observed in the summer of 1871. In the summer of 1874 I took an opportunity of studying Limneus in greater 
detail, and published an account of its embryology in the Quart. Journ. Microsc, Sci., October 1874, with two 
plates. 
